Core-magnet type instrument having linear response characteristic

ABSTRACT

A core magnet type instrument which renders a linear response characteristic comprises a magnet core, a yoke surrounding and spaced apart from the magnet core to form an air gap therebetween, and a moving coil movable in the air gap and having a pointer. The magnet core is shaped into an ellipse to improve a response characteristic of the instrument.

United States Patent 91 Inami et al.

[ 1 Jan. 16, 1973 CORE-MAGNET TYPE INSTRUMENT HAVING LINEAR RESPONSECHARACTERISTIC Inventors: Tetsuzo Inami; Yoshiyuki Takizawa,

both of Chichibu, Japan Assignees: Canon Kaliushiki Kaisha, Tokyo; CanonDenshi Kabushiki Kaisha, Chichibu-shi, Japan Filed: Nov. 16, 1970 Appl.No.: 89,687

Foreign Application Priority Data Nov. 17, 1969 Japan ..44 91947 US. Cl...324/151 A, 324/132 Int. Cl. ..G0ll l/16, G011 l5/l0 Field of Search.324/132, 154, 151, 151 A;

[56] References Cited UNITED STATES PATENTS 3,005,952 .l0/l96l Basingeru..3'24/151 R OTHER PUBLICATIONS .Bunn et al. Wireless World, December,1960, pp.

Primary Examiner-Alfred E. Smith Attorney-Ward, McElhannon, Brooks &Fitzpatrick [57] ABSTRACT 5 Claims, 21 Drawing Figures PRIOR ART HQ 4PATENTEDJAH 16 I973 Q 3.711.?76

sum 1 0P4 FIG. l B

PRIOR ART I 6 -29 I 'FIG. 2 FIG. 3

pmorz ART ART B(GAUSS) B(GAUSS) VmoR ART FIG 6 I FIG, 7 PRlOQ Al? yB(GAUSS) 5 v5 4 more ART PATENTEIJJAH 16 I975 3.711.776

sum 3 OF 4 FIG. l3 FIG; l4

8(GAUSS) i MAGNETIZING DIRECTION CORE-MAGNET TYPE INSTRUMENT HAVINGLINEAR RESPONSE CHARACTERISTIC The present invention relates to acore-magnet type instrument having a linear response characteristic andmore particularly to a core-magnet type instrument used as ammeter,voltmeter and the like.

In general, in the core-magnet type instrument used as ammeter,voltmeter and the like a moving coil is so arranged as to rotate aroundan internal magnet which is shaped into a cylindrical form at least overan angle of rotation of the moving coil, and the current or voltage tobe measured is applied to the moving coil so that the latter may bedeflected through an angle which may be read through a pointer attachedto the moving coil. In the instrument of the type described, it isessential that the moving coil responds linearly over a wide range ofthe variable to be measured.

In this specification, the linear response means that the moving coilrotates in linear proportion to the variable applied to the moving coil.Therefore when the pointer is attached to the moving coil, the angle ofdeflection of the pointer is in proportion to the variable beingmeasured so that the scale may be equidistantly graduated. For examplewhen the instrument of the type described is used as an exposure meterfor camera, it is desired that the instrument has a linear deflectionangle characteristic over a wide range of brightness because theexposure error may be reduced to the minimum throughout the measuringrange. In addition, the design of the instrument is much facilitated.For example the exposure meter may be easily coupled to otherinstrument. Furthermore the interchangeability of the graduated scale ofthe instrument may be attained so that the universal graduated scalesmay be employed in various instruments. In consequence it will be nolonger necessary to provide specific graduated scales for individualinstrument.

In practice, however, it is extremely difficult for the instrument ofthe type described to provide a linear response characteristic as willbe described in more detail hereinafter. In brief, in a moving-coil typeinstrument the relation between the current to be measured and an angleof deflection 9 of the moving coil is given by the following relation:

where B Flux density in the air gap between the internal magnet and theyoke;

b =.width of the moving coil;

1 length of the moving coil in the air gap between the internal magnetand the yoke;

n number of turns of the moving coil; and

1== coefficient of control of control spring. Since I, b, n and 1- areconstant, the angle of deflection becomes in proportion to the current ito be measured when B is made constant. However, it is extremelydifficult to obtain the uniform or constant flux density B throughout awide range of angle of deflection because when the internal magnet ismagnetized in a predetermined direction, the flux density is maximum inthat direction while in other directions the flux density is varied.

To overcome this problem there has been proposed a method for attachingthe compensators to the internal magnet in the direction ofmagnetization so that the variation in flux density through an angle maybe prevented. However, this method cannot overcome the problemcompletely as will be described in more detail hereinafter. In addition,the step of attaching the compensators to the internal magnet is addedin the manufacturing process and the desired characteristics will not beattained unless the compensators are attached to the internal magnetwithout any gap therebetween. This presents the serious problem in themanufacture. Furthermore, the compensators are employed in order toreduce the flux density in the direction of the magnetization so thatthe flux density as a whole is reduced, resulting in the low sensitivityof the instrument.

It is therefore one of the objects of the present invention to providean instrument having a linear response characteristic which mayeliminate the defect encountered in the prior art instrument.

It is another object of the present invention to provide an instrumenthaving a linear response characteristic in which the configuration ofthe internal magnet is made into an elliptical form at least over anangle of rotation of the moving coil.

It is a further object of the present invention to provide a magnet coreor internal magnet which is made by sinterring a ferromagnetic materialsuch as barium ferrite and has a linear response characteristic.

The present invention will become more apparent from the followingdescription of the preferred embodiments thereof taken in conjunctionwith the accompanying drawing in which:

FIG. 1(A) is a top view of the prior art instrument having a moving coiland a circular or cylindrical magnet core or internal magnet;

FIG. 1(B) is a side view partly in section of the instrument shown inFIG. I-(i);

FIGS. 2 and 3 are graphs showing the flux density distribution of theinstrument shown in FIG. 1;

FIG. 4 is a graph illustrating the relation between the current and theangle of deflection in the instrument shown in FIG. ll;

FIG. 5 is a cross section of the improved prior art instrument;

FIGS. 6 and 7 are graphs showing the flux density distribution of theprior art instrument shown in FIG. 5;

FIG. 8 is a graph illustrating the relation between the current and theangle of deflection in the prior art instrument shown in FIG. 5',

FIGS. 9 and 10 are graphs for explanation of the principle of thepresent invention;

FIG. 11 is a top view of an instrument incorporating the internal magnetor core in accordance with the present invention;

FIGS. 12 and 13 are graphs showing the flux density distribution of theinstrument shown in FIG. 11;

FIG. 14 is a graph illustrating the relation between the current and theangle of deflection in the instrument shown in FIG. 1 ll;

FIG. 15 is a diagram illustrating the relative position of the internalmagnet of the instrument shown in FIG. 1 1 with respect to its movingcoil; and

FIGS. 16 through 20 are graphs for explanation of the two examples ofthe present invention.

The prior art instrument will be described with reference to FIGS. 1-4.Referring first to FIG. 1, reference numeral 1 designates an internalmagnet; 2, a

moving coil which has a plurality of turns of windings securely held inposition by means of a suitable adhesive agent and is pivoted forrotation around the internal magnet 1', 3, a yoke; 4, a pointer madeintegral with the moving coil 3; 5 and 5', a pair of vertically spacedapart brackets for pivoting the moving coil therebetween; 6 and 6',control spring retaining arms fixed to the brackets 5 and 5respectively; 7, a O-adjustment arm extending from the retaining arm 6';and 8, spiral control springs loaded between the retaining arms 6 and 6'and the moving coil 2. It should be noted that the spiral springs areinsulatively fixed to the retaining arms 6 and 6' (the insulators arenot shown) and the current to be measured flows through the moving coil2.

When the internal magnet l is magnetized in the direction X in FIG.1(B), the distribution of the flux density between the internal magnet Iand the yoke 3 is indicated as shown in FIGS. 2 and 3. In FIG. 2, theorigin 0 is the center of the internal magnet 1; the X- axis is thedirection of magnetization; and the Y-axis is at a right angle relativeto the X-axis. The angle 0 is an angle of inclination relative to theX-axis. As viewed from FIG. 2, the distribution of the flux density isin the form of two circles and the flux density B in the direction at anangle 6 relative to the X-axis may be represented by the length betweenthe center 0 and the intersection of the straight directed line at anangle 0 with the circumference of the flux density distribution circle.From FIG. 2, the relation between the angle 0 and the density B may beplotted as shown in FIG. 3, and may be given by the following equation:

B=Bmcos0 where Bm flux density at 6 O, that is along the X- axis. Therelation between the current to be measured and the angle of deflectionin case the internal magnet 1 having the flux density distributioncharacteristic as described above may be illustrated in FIG. 4. FIG. 4shows the relation between the current indicated by i% (the maximumcurrent that the instrument can measure is I00 percent) and the angle ofdeflection 0 of the pointer 4.

When the cylindrical inner magnet is employed, the deflection anglecharacteristic is deviated from the theoretical curve indicated by thedotted line in FIG. 4. That is the non-linear curve in FIG. 4 is in theform ofS which intersects with the theoretical curve at 0 0, at anintermediate point of the range of the effective angle of deflection andat another end of the range thereof.

One of the prior art improvements which is intended to eliminate theabove described defect (the nonlinearlity of the curve of the angle ofdeflection) is illustrated in FIG. 5. The compensators 10 are affixed tothe internal magnet or core 11 in the direction of magnetization. Thecompensators 10 are made of a soft iron magnetic material. Because ofthe reluctance of the compensators l0 interposed between the yoke 13 andthe internal magnet 11, the flux density B in the air gap between theyoke 13 and the internal magnet 11 is reduced as shown in FIGS. 6 and 7.As viewed from FIG. 7 the flux density B in the directions adjacent to 00 is greatly reduced as compared with the case only the cylindricalinternal magnet l is used as shown in FIG. I. In consequence, thecurrent vs. angle of deflection characteristic may have the linearlityalmost along the range of effective angle of deflection as shown in FIG.8. However, the manufacture of the core assembly having the ideal linearresponse encounters difficulty because it is greatly affected by variousfactors such as the thickness of the compensators, the methods foraffixing them to the internal magnet and so on. In addition theprovision of the compensators will result in the decrease in sensitivityof the instrument.

According to the present invention, the internal magnet is shaped in theform of an ellipse in accordance with the theoretical equations to bedescribed in more detail hereinafter in order that the magnetic fluxlinked across the moving coil may be made uniform or constant all timesin the all range of the angle of deflection of the moving coil when theinternal magnet or core is magnetized in a predetermined direction.

Next the principle of the present invention will be described in detailhereinafter. As shown in FIG. 9, the center of a circular magnet core isthe origin 0; the direction of magnetization (toward the N pole) is inthe X-axis; and the Y-axis is at a right angle relative to the X-axis.The point P is a point arbitrarily selected outside of the circularmagnet core while the point B is a point arbitrarily selected along theperipheral edge of the circular magnet core. (fi= l (the distancebetween the center of the magnet core and the inner edge of the yoke),T37 rd), r,,, and m, is the strength of magnetic pole at the point ofmagnetization A. Then the strength of magnetic pole m4; at the point Bangularly spaced apart from the point A by 4) is given as follows:

o (I) Since the magnetic pole is at infinity, the magnetic potential atthe point P is given by From Eq. (2) it is seen that the strength ofmagnetic pole must be varied in order that the magnetic potential alongthe locus of the point P whose distance from the center 0 is maintainedconstant may be made equal independently of the angle 0.

The coordinate system employed in FIG. 10 is the same as in FIG. 9. Thedistance OA from the center 0 to the peripheral edge A angularly spacedapart from the X-axis by 0 is assumed to be r in order that the magneticpotential along the locus of the point P whose distance from the centerof the magnet core 0 is I may be made equal within the range of :01.Then the magnetic potential U, at the point P in FIG. 10 and themagnetic potential U, at the point P are U',,= Ua+ V, 4 where V, and V,are the magnetic potentials at angles except 0 and a. Since V, isapproximately equal to V,,, Eqs. (3) and (4) are made equal when U Ua Onthe other hand, U and Ua .are given by That is the problem is to obtainr for making Eqs. (5) and (6) equal.

m a (8) Therefore, substituting Eqs. (7) and (8) in Eqs. (5) and (6),and making U U we have m, cosa cosO/l-r r /r,,,=cos ell-r Hence,

r9 =l(C/1+C) where C= r,,,/l r,, cosa/cos 0 the origin 0 and theintersection between the curve a and the side edge of the moving coil isnot used in a strict sense. Therefore the actual effective angle of Thismeans that the flux density curve must have a flat top within an angleof 150 20:).

It is seen that except the configuration of the magnet core the designsof the moving coil, the control springs,

the pointer and so on may be made as with the case of the conventionalinstruments having 'the moving coils. When the pointer is fixed to thecoil, the uniformly graduated scale may be employed.

The two examples of the instruments A and B based Thus the length of themagnetic core r9 at an angle of "P the Principle explained above will bedescribed 0 may be derived from Eq. (10). In addition, it is seen fromEq. (10) that the radius r in the direction of 0 of the magnetic coremay be determined in combination with the radius r,,, of the magneticcore to be manufactured, the inner radius of the yoke I and theeffective angle of deflection of the pointer.

The configuration of the magnet core which satisfies Eq. (10) issubstantially an ellipse designated by 21 in FIG. 11 and thedistribution of the magnetic flux density is shown in FIGS. 12 and 13.In FIG. 12 the flux density B is the vector length between the origin 0and the curve a and the flux density in the direction of magnetizationbecomes more uniform as compared with with reference to FIGS. 16-20hereinafter.

(I) Instrument A Diameter of magnet core: 10 mm 5 Radius of yoke: l2.8mm

Effective angle B of deflection: 60

2 0: (angle not used because of the of moving coil): 40

(II) Instrument B Diameter of magnet core: 10 mm Radius of yoke: 12.8 mmEffective angle B 75 the circular magnet core indicated by the brokenline b.

is exactly in proportion to the angle of deflection of the pointer.

The circular or cylindrical magnet cores were previously prepared forboth of the instruments A and B and the distances from the center of thecylindrical core tothe peripheral edge were calculated as shown in TableI.

6 0 10 20 30 40 S0 A 4.46 4.48 4.57 4.65 4.80 5.00 B 4.10 4.12 4.19 4.314.48 4.71 5.00

Table I: Distances from the centers of the cores to their peripheries tosatisfy Eq. (10), unit mm. The configurations of the magnet cores forthe instruments A and B shaped in accordance with Table 1 areillustrated in FIG. 16 by c and d respectively. The flux densitydistributions are shown in Tables 2 and 3 and in FIGS. 17 and 18respectively.

'raiaie iQinsTnun/ian r 1; a=50 The reason why the effective range ofangle of deflection B is smaller than the angle a will be at once notedfrom FIG. 15. Since the moving coil has the width w as described above,the angle -y between the center line of the moving coil and the lineconnecting [B 10 Gauss] TABLE 3.INS'IRUMENT 13;a=60

113x10 Gauss] where 0 an angle between the reference line passing 65through Nos. 1, 2 and 3 designate the cores provided in a similarmanner. FIGS. 17 and 18 illustrate the average values of these threecores.

The current vs. angle of deflection characteristics of said magnet coreis magnetized in a single predetermined direction relative to said yoke;and

the configuration of said magnet core satisfies the following equationat least within an angle of rotathe instruments A and B are illustratedin Table 4 and 5 5 tion of said moving coil:

and FIGS. 19 and 20 respectively, from which it is seen .1,

that the ideal linear proportionality is attained. r =1 C/l C i $4131.184.-1NsTRUMEN i .4; 60 meter 7' No.1[l(percent 8.87 17.28 25.43 33.8541.9 50.32 58.46 66. 52 74. 66 86. 78 91. 86 100 N0. 21(percent 9.4918.34 26.73 35.02 43.32 51.52 59.54 67. 37 75.48 8.34 92.16 100 NO.31(percent 8.75 17.5 25.9 33.93 41.6 49.37 57.85 66.07 74.1 82.5 91.5100 I 1.1618 5. 1N s'112b114E NT B; 75 166661 No.1l(percent 7.05 13.9720.7 27.3 34.3 40.83 47.31 53. 78 60.3 67.0 73.02 79.06 85.72 92.7 No.2{i(perc9nt 7.48 14.6 21.24 28.1 34. 77 41.1 47.97 54.66 61.41 68.04 74.17 80.26 86.34 93.0 100 'No. 31(percent 65 14 66 21.47 28.07 34.52 41.33 47.86 54.45 60.98 67.4 74. 03 80.34 86.66 92. 97 100 FIGS. 19 and 20illustrate the average values of the where C =r,,,/lr,,, cos X/cosOthree cores (Nos. l-3) when incorporated in the same instrument r lengthof a directed [me at an angle of 0 with The magnet core in accordancewith the present in- 25 respect to the single dlfecnon P mag'netlzaflonvention may be produced by molding a ferromagnetic from h centerrotation of Said moving coll to material such as barium ferrite in amold having a cavih p p F of P magnet core; ty shaped in accordancewith, for example,Table 1. Al- FHQE BQH 9f5.' l .}9 ternatively,suitable magnets may be machined into a a (effeqwe angle of deflectfonmovmg desired shape 30 coll plus twlce the angle of moving coil width);and

From the foregoing it is seen that when the magnet rm length of a Q F atf g of X i core is shaped into an elliptical form in accordance respectto the dkrecnon of ,magnetlzatlon f sand with the present invention, theideal linear proporcenfer of rotalnon of Sand movmg coll to thetionality may be attained between the current i and the Penphery coreangle of deflection 0. In consequence, the prior art 35 Sald core bemgfi respect to Sam yolfe compensators attached to the core may beeliminated. member and Send gap foYmed P f f The magnet cores inaccordance with the present in- Y and sa1d core bemamamtamed vention maybe produced easily by a suitable process umform magmt1C field over angleof rota such as sinterring without the problems of the dimen- H v V Isional tolerances and eccentricity. Furthermore, the in- 40 2. A coremagnet type instrument according to claim struments incorporating themagnet cores in ac- 1 wherein said magnet core is shaped substantiallyinto cordance with the present invention may be manufacan ellipse; andsaid moving coil has a side whose length tured without increasing theassembly steps and the like is at least equal to the major axis of saidellipse. as In the case of the prior art instrument. In addition, A coremagnet p instrument according to claim when the mstliumem 'P magnet 45 1wherein said yoke member is made of a cylindrical f F P mvemlon Samemagnet material and assembled in unitary construction W ththe prior artinstrument, the magnet ic flux density with said magnet comdistributionmay be more effectively utilized without 4 A t any loss.This advantage is essentially remarkable in W core type msirumemaccordmg to 9 case of the large sized instrument 50 1 wherein said coremagnet IS a magnet made by sinter- What is claimed is: ring bariumferrite.

l. A core magnet type instrument having a linear 5. A magnet core typeinstrument according to claim response characteristic, comprising amagnet core, a 1 wherein a pointer is fixed to said moving coil and isyoke member spaced apart from said core so as to form deflectedequiangularly when the current flOWS through an air gap therebetween anda moving coil movable in 55 Sai m ing Coil. said air gap: characterizedin that 4 =1 I UNHED STATES PATENT GFFKQE 9F CREQTEQN I Patent No.3,711, 776 7 Y I Dated January 15 1213 7 I lnveniofls) TE'PSUZO INAMI eta1 It is. certified that error appears. in' the above-identified patentarid that said Letters Patent vare herebycorrectd as s'hownbelow:

' Column 3, line 5; for "3" to-read 2-;

Column 8,.TABLE line 4 from'."8.34" to read --83.4--;

Signed and sealed this 10th ay of July 1973.

(SEAL) Atte st EDWARD MQFLETCHERQJR; Rne y Attesting Officer A tiCommissioner of Patents"

1. A core magnet type instrument having a linear responsecharacteristic, comprising a magnet core, a yoke member spaced apartfrom said core so as to form an air gap therebetween and a moving coilmovable in said air gap: characterized in that said magnet core ismagnetized in a single predetermined direction relative to said yoke;and the configuration of said magnet core satisfies the followingequation at least within an angle of rotation of said moving coil: r lC/1 + C where C rm/l - rm . cos X/cos theta r length of a directed lineat an angle of theta with respect to the single direction ofmagnetization from the center of rotation of said moving coil to theperiphery of said magnet core; l inner radius of said yoke; Alpha 1/2(effective angle of deflection of said moving coil plus twice the angleof moving coil width); and rm length of a directed line at an angle of Xwith respect to the direction of magnetization from said center ofrotation of said moving coil to the periphery of said magnet core, saidcore being fixed with respect to said yoke member and said air gapformed between said yoke member and said core being maintained with auniform magnetic field over said angle of rotation.
 2. A core magnettype instrument according to claim 1 wherein said magnet core is shapedsubstantially into an ellipse; and said moving coil has a side whoselength is at least equal to the major axis of said ellipse.
 3. A coremagnet type instrument according to claim 1 wherein said yoke member ismade of a cylindrical magnet material and assembled in unitaryconstruction with said magnet core.
 4. A magnet core type instrumentaccording to claim 1 wherein said core magnet is a magnet made bysinterring barium ferrite.
 5. A magnet core type instrument according toclaim 1 wherein a pointer is fixed to said moving coil and is deflectedequiangularly when the current flows through said moving coil.